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In the study of dye-sensitized solar cells (DSCs), developing computational predictions of key properties of the dyes is highly desirable for identifying promising candidates. In this work, we report first-principle-based, theoretically estimated charge-recombination (CR) rates from TiO2 to the cationic dye for 2 pairs of organic dye molecules. A recently developed multistate fragment charge difference (msFCD) scheme, together with long-range-corrected time-dependent density functional theory, was used to calculate the electronic coupling. The msFCD scheme removes the local excitation components in the charge-transfer states, and generates acceptable diabatic states. The range-separated ωPBE and BNL functionals were useful for the charge-transfer problem, and results were largely independent of TiO2–dye binding modes. The rates obtained for CR to oxidized dye (CRD) followed a trend similar to experimental results. In general, a difference in the reorganization energy (λ) and the free energy (ΔG0) had a la...

Photovoltaic devices have great potential in harvesting solar energy. In organic solar cells, the role of the donor–acceptor heterojunction is critically important; here optical excitation leads to charge separation (CS) and subsequently photocurrent generation. Charge recombination (CR) may also happen, which reduces the overall efficiency of the device. After light absorption, the singlet excitation of a pentacene can achieve CS at the interface, or it may also undergo singlet fission and produce two triplet excitons, which may further undergo CS at the interface. In this work, we study charge-transfer processes at the pentacene–C60 interface, including CS rates in both singlet and triplet surfaces, and the CR rate back to the ground state. We first constructed two different pentacene–C60 interfacial structures by force field optimization. With pentacene–C60 molecular pairs derived from the structures, we calculate the electronic couplings of CS and CR reactions. We found that the electron-transfer coup...

The energy-transfer properties of a regioregular silylene-spaced alternating donor–acceptor copolymer [(donor)3-SiMe2–acceptor-SiMe2]m (denoted (D3A)m) are determined with time-resolved spectroscopy; 4,4′-divinylbiphenyl serves as the donor and 4,4′-divinylstilbene as the acceptor. With excitation at wavelength 266 nm, fluorescence up-conversion curves at various detection wavelengths are measured. The observed decay of donor emission at a time constant 0.3 ps and the emission of the acceptor have a corresponding fast rise and a slower second rise at ∼2 ps. In the tail of fluorescence emission, the varied slow decay components (2–7 ps) on the donor and rise on the acceptor are observed and assigned to intrachain aggregates. With the Coulomb coupling which is beyond the multipole approximation, the Fermi golden rule rates of energy transfer are calculated between adjacent monomers and between the D2 exciton and A. In all D2A and D3A conformers calculated, we found that the coupling strengths of D–A and D–D...

The synthesis and properties of carbazole and diphenylamine derivatives with different numbers and positions of methoxy groups in the diphenylamino moiety are reported. A comparative study on their thermal, optical, and photoelectrical properties is presented. All the synthesized compounds are found to form glasses with the glass transition temperatures in the range of 39−69 °C, as characterized by differential scanning calorimetry. The ionization potentials of the synthesized compounds were estimated both theoretically by quantum chemical calculations and experimentally by an electron photoemission in air technique. The trends observed by theoretical calculations are confirmed by experimental results. The experimental values of ionization potentials range from 5.10 to 5.56 eV. Compounds containing monomethoxy-substituted phenyl rings exhibited lower ionization potentials than compounds containing di- and trimethoxy-substituted phenyl moieties. The highest ionization potential was observed for the compoun...

The energy-transfer properties of a regioregular silylene-spaced alternating donor−acceptor copolymer (donor−SiMe2−acceptor−SiMe2)m are determined with time-resolved spectroscopy; 4,4′-divinylbiphenyl serves as donor and 4,4′-divinylstilbene as acceptor. Steady-state fluorescence spectra show that the energy transfer is efficient. With excitation at 266 nm, fluorescence up-conversion curves at various detection wavelengths are measured to explore the detailed dynamics of the energy transfer. A rate ∼(0.3 ps)−1 for energy transfer between the S1 states of the donor and acceptor moieties is observed. Theoretical calculations based on fragment excitation difference provide a rate of energy transfer near the experimental value with detailed insight into the mechanism of energy transfer. The results of theoretical calculations indicate that the Coulomb interaction is dominant and the Dexter process is weak. The high-multipole interactions are important for energy transfer.

Series of dyes containing a cyanovinyl entity in the spacer have been developed and characterized. Dye-sensitized solar cells (DSSCs) using the dyes as the sensitizers exhibited good efficiencies ranging from 3.48−4.92%, which reached 45−70% with respect to that of an N719-based device fabricated under similar conditions. In general, better DSSC performances are observed with molecules having a shorter distance between the internal cyanovinyl entity and the amine donor. Computational analysis indicated that the cyanovinyl entity may behave as a charge trap, especially in the long molecules studied, where the cyanovinyl group is away from the electron-donating amine by two or more aromatic groups. A positive correlation is observed between the theoretically calculated product of oscillator strengths and the charge shift at the cyanoacrylic acid group and the measured short-circuit current in DSSCs. Therefore, it is likely that the charge-trapping in the cyanovinyl entity reduces the electronic interaction ...

The electronic coupling in excitation energy transfer (EET) is composed of a Coulomb coupling (which can be approximated as the Forster's dipole coupling), a Dexter's exchange coupling, and a term arising from the orbital overlap of the donor and acceptor fragments. We have developed a new scheme to account for the EET coupling by generalizing the fragment charge difference scheme [Voityuk, A. A.; Rosch, N. J. Chem. Phys. 2002, 117, 5607] for electron-transfer coupling. As a result, the EET coupling can be calculated in a general class of systems irrespective of the molecular symmetry. The short-range coupling, defined as the contribution from Dexter's exchange coupling and the overlap effect, was obtained as the difference of the EET coupling from this new scheme and a precise account of the Coulomb coupling. For a pair of stacked naphthalenes, the short-range coupling is very similar to the triplet−triplet energy-transfer coupling in both magnitudes and the distance dependences. We have observed cases i...

Series of dyes based on 1H-phenanthro[9,10-d]imidazole chromophores were developed and characterized. Dye-sensitized solar cells (DSSCs) using the dyes as the sensitizers exhibited good efficiencies, ranging from 2.95 to 4.68%, which reached 45−70% with respect to that of an N719-based device fabricated under similar conditions. Incorporation of arylamine moieties at 1H-phenanthro[9,10-d]imidazole was found to retard charge recombination of the electrons in the conduction band of TiO2 with the oxidized dyes. In contrast, incorporation of hydrophobic dodecanyl chains at 1H-phenanthro[9,10-d]imidazole reduced the dark current and improved the device efficiency. Results of TDDFT computation on the compounds are in support of the charge-transfer character of the compounds and consistent with the device performance. A cascade of charge separation is possible for compounds with arylamine moieties via an internal conversion from a bright higher excited state to a long-range charge separated state. Our results in...